Hydrocarbon synthesis



Patented Apr. 12, 1949 2, 467,282 nYnaocARnoN SYNTHESIS Benjamin R.Warner, Chicago, Ill., assignor to Gull Research 85 Development Company,Pittsburgh, Pa., a corporation of Delaware No-Drawing. Originalapplication February 27,

1947, Serial No. 731,403. Divided and this application May 19, 1948,Serial No. 28,060

This invention relates to hydrocarbon synthesis, and in particularconcerns a process for synthesizing hydrocarbons from hydrogen andcarbon monoxide in the presence of a catalyst.

In the commercial synthesis of hydrocarbons by the so-calledFischer-Tropsch process, whereby a mixture of hydrogen and carbonmonoxide is contacted with a catalyst at an elevated temperature, thecatalysts have usually comprised cobalt with the addition of variousactivators such as thoria and the like. Although other metals suitableas catalysts for the synthesis are known, e. g., iron, nickel andothers, the greater activity of cobalt catalysts has heretoforemilitated against the extensive use of such other metals. However,despite the greater activity of cobalt catalysts, the processes based ontheir use are not entirely desirable from a commercial standpoint'sincethe great expense involved in the preparation of such catalystsnecessitates elaborate and costly systems for recovering spent catalystvalues after the catalyst has lost its activity during the course of theprocess. Furthermore, the cobalt catalysts have the disadvantage ofproducing predominately straightchain saturated hydrocarbons of lowoctane number in the motor fuel boiling range, as well as excessiveamounts of high molecular weight wax-like hydrocarbons which becomedeposited on the catalyst during the course of the process. The latterphenomenon reduces the activity of the catalyst and necessitatesfrequent removal of the wax-like hydrocarbons to permit of continuedcatalyst activity over any substantial length of time. The necessity ofremoving the wax requires the obviously disadvantageous procedure ofinterrupting the process at periodic intervals. 1

It has long been desirable to employ a hydrocarbon synthesis process inwhich a metal less expensive than cobalt is used as the catalyst. Tothis, end, attempts have been made to use finely divided iron. A processemploying a catalyst of this nature would not require elaborate andcostly recovery procedures for the deactivated catalyst since, in theordinary course. when the iron catalyst becomes deactivated, it issufliciently cheap simply to be discarded. However, the iron catalystsheretofore known have not possessed sufficiently great activity towarrant their commercial use for the synthesis of hydrocarbons.Furthermore, it has been typical of such iron catalysts that a largeproportion of the carbon monoxide contained in the synthesis gas isconverted during the process to car- 5 Claims. (Cl. 260-44913) bondioxide instead of to the desired valuable hydrocarbons.

It is accordingly an object of the present inhydrocarbons from hydrogenand carbon monoxide employing an iron catalyst which com-' paresfavorably in activity with cobalt catalysts.

Another object is to provide a hydrocarbon synthesis process whichemploys an iron catalyst and which does not result in the production ofexcessive amounts of high molecular weight waxlike hydrocarbons.

A further object is to provide an iron catalyzed hydrocarbon synthesisprocess which produces hydrocarbons boiling in the motor fuel range andhaving a higher octane number than those obtained from the use of cobaltcatalysts. A still further object is to provide an iron catalyzedhydrocarbon synthesis process which does not produce excessive amountsof carbon dioxide.

Other objects will be apparent from the following detailed descriptionof the invention, and various advantages not specifically. referred toherein will occur to those skilled in the art upon employment of theinvention in practice.

I have now .found that the above and related objects may be realized ina process whereby a mixture of hydrogen and carbon monoxide is contactedat an elevated temperature with a precipitatediron catalyst which hasbeen prepared by adding to a water-soluble iron salt a watersolublealkaline compound to the point of incipient precipitation and thenadding gallic acid to complete the precipitation. Such catalyst may beof the supported or unsupported type, and

may contain activators as hereinafter more fully explained.

In preparing the catalyst employed in the new process, a water-solublealkaline compound, such as potassium carbonate, is added with stirringto a warm aqueous solution of a water-soluble iron salt, such as ferricnitrate, to the point of incipient precipitatiom At this point, anaqueous solution of gallic acid is added with stirring and precipitationtakes place. The precipitate is then filtered, washed with water anddried. The resulting product may be used directly as the catalyst in thesynthesis process, in which case it will become reduced prior to anysynthesis reaction occurring, or it may be reduced by treatmentwithhydrogen in any suitable manner prior to use in the process.

The manner in which the gallic acid acts to complete the precipitationand the method by which it aflects the precipitate are not fullyunderstood. While I do not desire to be limited by any theory as to theaction of the gallic acid, it is my present belief that the gallic acidis either adsorbed on the precipitate or reacts therewith in some mannerto produce an unusual form of catalyst. In any event, it is certain thatthe gallic acid produces profound changes in the nature of the catalystas evidenced by its enhanced activity and lower carbon dioxideproduction in the synthesis. Furthermore, X-ray examination of ironcatalysts after use in the synthesis of hydrocarbons shows that an ironcatalyst prepared without gallic acid contains iron carbide and carbonand is not pyrophoric; whereas an iron catalyst prepared in accordancewith my invention shows little iron carbide, no carbon, and is extremelypyrophoric.

Any water-soluble iron salt may be used as a starting material in thepreparation of the catalyst. Either ferric or ferrous salts may be used.In general, the ferric salts are preferred because they are cheaper andmore stable than the ferrous salts. Ferric nitrate is preferred amongthe ferric salts because the nitrate ion. is more readily washed out ofthe precipitated iron compound and is decomposed at the temperatures ofeither the reduction or synthesis. Traces of sulfate and chloride ion onthe finished catalyst tend to act as poisons, and therefore the use ofiron sulfates and chlorides as starting materials in the preparation ofthe catalyst is not preferred because sulfate and chloride ions aredifflcult to remove completely.

Any water-soluble alkaline compound, that is, a compound which willyield hydroxyl ions in aqueous solution, may be used to bring the ironsalt to the point of incipient precipitation. For example, the alkalimetal hydroxides, such as sodium and potassium hydroxides, the alkalimetal carbonates, such as sodium and potassium carbonates, ammoniumhydroxide and the watersoluble amines, such as triethanolamine, aresuitable.

As previously stated, the water-soluble alkaline compound is added tothe water-soluble iron salt in an amount sufficient to produce incipientbut not complete precipitation. The exact amount of water-solublealkaline compound to add to produce incipient precipitation may varydepending on whether a ferrous or ferric salt is used as the startingmaterial. Regardless of the type of iron salt used, however, thewater-soluble alkaline compound should not be added in any substantialexcess of the amount theoretically necessary to precipitate ironhydroxide. For example, when the iron salt is a ferric salt, thealkaline compound is added in such an amount that the pH of theresulting solution lies in the range 2.1 to 2.3. When the iron salt is aferrous salt, the alkaline compound is added in such an amount that thepH of the resulting solution lies in the range 5.3 to 5.5,

The amount of gallic acid added when the mixture of water-soluble ironsalt and water-soluble alkaline compound is at the point of incipientprecipitation is such that the iron compound is completely precipitatedfrom the mixture. In general, at least 0.08 mol of gallic acid per molof iron salt used as starting material is required.

Although large amounts of gallic acid may be used, no advantage isordinarily obtained thereby, and I prefer not to use more than 0.15 molof gallic acid per mol of iron salt. When further amounts of alkalinecompound are added after precipitation of the iron compound toprecipitate activator compounds, as hereinafter disclosed, the largeramounts of gallic acid are preferably used to offset any possibleconsumption of gallic acid by the alkaline compound.

As previously mentioned, various promoters or activators may be added tothe iron catalyst and, in addition, various supporting materials for thefinished catalyst, such as kieselguhr, silica gel and the like, may beused. The activators and the supporting materials are conveniently addedto the water-soluble iron salt prior to addition of the water-solublealkaline compound. In such case, the activator is also added in the formof a water-soluble salt. When it is desired to incorporate an activatorwhich is to be precipitated by the water-soluble alkaline compound, itis preferred to complete the precipitation of the iron compound first inthe manner described' above, that is, by the addition of a water-solublealkaline compound to the point of incipient precipitation followed bythe addition of gallic acid to complete the precipitation of the ironcompound, and then to add additional water-soluble alkaline compound toprecipitate-the activator. In such case, greater amounts of gallic acid,as set forth hereinabove, are preferably used to precipitate the ironcompound, in order to offset any leaching effect on the gallic acidoccasioned by the addition of further amounts of water-soluble alkalinecompound after the gallic acid has been added.

Among the activators which may be used are manganous oxide, copper,magnesium oxide and calcium oxide. The activators are generally used ina minor amount, and the proportion of activator to iron metal in thereduced catalyst may vary from about 1 per cent to 25 per cent. Theamount of activator used in any specific instance will vary depending onthe specific activator chosen. Also more than one activator may beemployed.

The following examples will illustrate the preparation of the catalystsemployed in the process of the invention. Unless otherwise stated, allparts are by weight.

Example 1.To 607' parts of Fe(NOa)s.9H2O dissolved in 3000 parts ofwater and heated between about 70 and 0., there Was added, withstirring, a solution of 290 parts of potassium carbonate dissolved in450 parts of water, At this point of incipient precipitation, 15 partsof gallic acid dissolved in 1000 parts of hot water were added andprecipitation took place. The precipitate was filtered, washed withwater until the washing showed negligible nitrate content, and wasoven-dried at C. As previously stated, carriers or supporting materialsfor the finished catalyst, such as kieselguhr, may be added to thesolution prior to precipitation. Other water-soluble alkaline compounds,such as ammonium hydroxide, may be used in lieu of the potassiumcarbonate.

Example 2.-To a hot solution, between about 70 and 90 C., of 607 partsof Fe(NOa)3.9H2O and 28 parts of Mn(NO3)z dissolved in 3000 parts ofwater, there was added, with stirring, a solution of 290 parts ofpotassium carbonate in 450 parts of water. At this point of incipientprecipitation, 30 parts of gallic acid dissolved in 1000 parts of hotwater was added and precipitation of the iron compound took place. Atthis point, the pH of the mixture is on the acid side and the manganouscompound does not precipitate. In order to precipitate the manganouscompound which'becomes the activator an additional 50 parts of potassiumcarbonate dissolved in 500 parts of water was added. The precipitate wasfiltered and washed with hot water until it showed negligible nitratecontent. The catalyst was then dried at 110 C. The larger amount ofgallic acid shown in this example is used, in accordance with theprevious disclosure, to offset the leaching effect of the additionalwater-soluble alkaline compound used to precipitate the activator.

As previously stated, the catalysts employed in accordance with myinvention may be reduced with hydrogen prior to use in synthesis, orthey may be reduced directly in the synthesis operation. Reduction ofthe catalyst prior to the synthesis may be accomplished in any knownmanner, as for example, by passing hydrogen over the catalyst at 500 C.

As previously stated the process of the invention consists essentiallyin contacting a mixture of hydrogen and carbon monoxide with thehereinbefore described catalyst at an elevated reaction temperature. Theprocess may be carried out in any suitable manner employing any of theknown engineering techniques. Thus, the mixture of hydrogen and carbonmonoxide may be passed over thin layers of the catalyst in a reac-' tor,and the products of the synthesis recovered from the exit gases. Ifdesired, the fluidized catalyst technique may be used, wherein thecatalyst, in finely divided form, is suspended in a stream of thesynthesis gas mixture and passed into a reactor. The catalyst may alsobe suspended in an inert liquid reaction medium into which the synthesisgas mixture is passed.

The reaction conditions for the synthesis may vary widely. In general,the temperature ranges from about 200 to 350 C.; the pressure may varyfrom 1 atmosphere to about 10 atmospheres; the space velocity (volumesof synthesis gas per volume of catalyst per hour) may range from about100 to 400; and the mol ratio of hydrogen to car bon monoxide in thesynthesis gas mixture may vary from about 2:1 to 1:1. If desired,unreacted synthesis gas mixture may be recycled to the reactor. Thecatalysts may be used in finely divided form or they may be agglomeratedor pelleted, if desired.

The following examples will illustrate the process of the invention, butare not to be construed as limiting the same.

Example 3.--A synthesis gas mixture of hydrogen and carbon monoxide in amol ratio of 1.9:1 was passed into a reactor over a reduced catalystprepared by the method of Example 1 at a temperature of about 340 C. andat a space velocity about 156. Under these conditions, 96 per cent ofthe carbon monoxide in the synthesis gas was converted. A yield of C2and higher hydrocarbons was obtained amounting to 131 grams percubicmeter of the synthesis gas. This compares favorably with previous cobaltcatalysts which have yielded approximately 130 grams of similarbydrocarbons per cubic meter of synthesis gas.

Example 4.In order to determine the relative activity of catalystsprepared with and without the use of gallic acid, the followingsyntheses were performed using in both cases Fe-MnO catalysts. one ofwhich was prepared in accordance with Example 2 and the other or whichwas similarly prepared except that no gallic acid was used. Thesynthesis was conducted at a temperature of 260 C. using a mol ratio ofhydrogen to carbon monoxide of about 1.8:1 and a space velocity of about160. The space-time-yield, that is, the volume of carbon monoxidereacted per volume of catalyst per hour, was 19 for the gallic acidcatalyst and only 10.5 for the catalyst prepared without gallic acid.

As shown by Example 4 above, the process of the invention secures aspace-time-yield almost twice as great as that attained in a processwhich employs an iron catalyst prepared in the usual manner. The newprocess has-the further advantage of yielding largely liquid products ofhigh octane number. In this respect it is markedly superior to the knownprocesses which employ cobalt catalysts since the latter produce largeamounts of wax-like hydrocarbons. Such products are not only injuriousto the synthesis in that they reduce the activity of the catalyst, butare not as valuable as the liquid products of the synthesis. The newprocess is similarly advantageous in that it does not produce excessiveamounts of carbon dioxide.

Other modes of applying the principle of my invention may be employedinstead of those explained, change being made as regards the materialsor procedure employed, provided the step or steps stated by any of theappended claims, or the equivalent of such step or steps, be employed.This application is a division of my copending application, Serial No.731,403, filed February 27, 1947.

I, therefore, particularly point out and distinctly claim as myinvention:

1. A process for the synthesis of hydrocarbons from hydrogen and carbonmonoxide which comprises passing a mixture of hydrogen and carbonmonoxide over a catalyst prepared by adding a water-soluble alkalinecompound to an aqueous solution of a water-soluble iron salt to thepoint of incipient precipitation, then adding gallic acid to theresulting mixture to complete the precipitation, and recovering theresultant precipitate.

2. A process for the synthesis of hydrocarbons from hydrogen and carbonmonoxide which comprises passing a mixture of hydrogen and carbonmonoxide over a catalyst prepared by adding a water-soluble alkalinecompound to an aqueous solution of a water-soluble'ferric salt, theamount of water-soluble alkaline compound being such that the pH of theresulting solution lies in the range 2.1 to 2.3, then adding gallic acidto the resultant mixture in such amount that the. mol ratio of gallicacid to ferric salt is not less than 0.08:1 thereby obtaining aprecipitate, and recovering the resultant precipitate.

3. The process of claim 2, wherein the catalyst is reduced prior topassing the mixture of hydrogen and carbon monoxide thereover.

4. The process of claim 2, wherein the mol ratio of hydrogen to carbonmonoxide in the mixture is from about 2:1 to about 1:1.

5. The process of claim 4, wherein the reaction temperature is fromabout 200 C. to about 350 C.

BENJAMIN R. WARNER.

No references cited.

